The present invention relates to a tightly sealed gas discharge lamp filled with a nitrogen oxygen mixture, the gas being held in the lamp at low pressure; an electric discharge current flows through such a lamp during operation.
Gas discharge lamps of the kind to which the invention pertains are used for example in photometers for determining, i.e. measuring the concentration of components in a gas mixture. For example, nitric oxide (NO) or sulfur dioxide SO.sub.2 absorb light in the range of ultraviolet radiation from 200 to 600 nm emitted from such a lamp. Usually the gas discharge lamps are constructed as hollow cathode lamps.
German patent 25 41 162 discloses a photometer for measuring the concentration of nitric oxide. This photometer includes a gas discharge lamp constructed as a hollow cathode lamp filled with low pressure air and operated with a low level discharge current. Further included is a radiation detector for receiving the nitric oxide resonance radiation emitted by the lamp after such radiation has passed through the gas mixture. The lamp consists of a vessel containing gas which is tightly sealed and under pressure of from 1 to 5 mbar. Such a hollow cathode lamp has a very short use life of its nitric oxide radiation owing to the consumption of oxygen of the air filling so that such a lamp is useful at best in laboratories but without further features cannot be used in a practical sense in industrial applications. Although increasing the air pressure does increase the use life of the nitric oxide radiation, the intensity of that radiation is reduced.
German patent 25 46 565 discloses another photometer that also uses a hollow cathode lamp as gas discharge lamp and radiation source. This particular photometer is primarily designed for measuring the content of sulfur dioxide in a gas mixture. However, certain features of that photometer permit its use for additionally measuring the content of nitric oxide in a gas mixture. The hollow cathode lamp is filled here with a dry nitrogen oxygen mixture, and after falling it is closed off by a melting and sealing process. The use life of this lamp is likewise short as far as nitric oxide radiation is concerned, particularly for measuring the concentration of nitric oxide, owing to the consumption of oxygen during operation of the lamp so that again such a lamp can be used only in lab- equipment; additional features are needed to expand its use to industrial applications.
It has thus been recognized that the low or short use life of the nitric oxide radiation of a hollow cathode lamp as known from the literature above is the result of the oxygen consumption and that is the reason for limiting industrial applications; this effect is particularly noticeable when using an ultra violet spectral range from 200 to 300 nm. In order to offset this limitation, replenishing the oxygen content by means of an oxygen-releasing substance has been tried. There was the underlying belief that oxygen can be produced by the decomposition of metal oxides such as manganese dioxide. The above-cited German printed patent application discloses such a lamp. This particular hollow cathode lamp is filled with a nitrogen oxygen mixture at a low pressure and is gas tightly sealed. The inclusion of a certain amount of metal oxide in the lamp is supposed to offset to some extent the gas consumption. However, it was found that the oxygen yield at the operating temperature of that lamp is too small and will not sufficiently replenish the oxygen consumption. The same is true as far as the effect of the low pressure in the lamp is concerned; it just does not suffice to do the job.
Additionally reference is made to U.S. Pat. No. 3,947,685 being a kind of continuation case for German patent 22 46 365.